WASHINGTON: Only about 11 per cent of so-called “insider attacks” by Afghans against Nato troops are due to Taliban infiltration, the Pentagon said on Friday, as more American troops were killed at the hands of Afghans posing as allies in the 11-year-old war.

The vast majority of insider attacks this year -- nearly 90 per cent -- are due to other motives, including personal grudges, Pentagon spokesman Lieutenant Colonel Steven Warren said, citing a new study by the Nato-led forces in Afghanistan.

The data challenge Taliban assertions of widespread infiltration of Afghan security forces, but also raise questions about the limits of Nato efforts to weed out insider threats as they prepare to hand over lead security control of the country to the Afghans in 2014.

The Pentagon announced earlier this week it was expanding counterintelligence staff in Afghanistan after a rise in insider attacks by Afghans, including attempts to spot signs of Afghan forces becoming radicalised without contact with insurgents.

But that would be little help in preventing grudge-related killings. And the number of attacks has been rising.

As of Tuesday, there had been 29 attacks so far in 2012, resulting in 37 deaths among the Nato-led coalition forces, 21 of which were Americans, according to Pentagon data. For the same period last year, there were 16 attacks and 28 deaths.

In all of 2011, there were 35 coalition troops killed, 24 of whom were US troops.

The latest incident on Friday came in Afghanistan's western Farah province, where a recently recruited police militia member in his late 60s killed two US troops shortly after being given his weapon at an inauguration ceremony, a US official said.

The motives of the killings were still unclear and an investigation was underway, Nato-led forces said.

General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, noted that Afghan forces had so far discharged hundreds of soldiers who showed a risk of radicalisation -- including travel back and forth to Pakistan, where many militants enjoy safe haven.

Reclusive Afghan Taliban leader Mullah Mohammad Omar has said insurgent fighters have successfully infiltrated the Afghan security forces. In a statement released late on Thursday, Mullah Omar urged police, soldiers and government workers to “abandon support of the invaders” and back the Taliban ahead of the departure of most Western combat troops in 2014.